FIG. 17 is a perspective view schematically showing the configuration of a conventional optical print head disclosed in, for example, Japanese Patent Application Laid-open (JP-A) No. 7-256928. As shown FIG. 17, white light emitted from a point-source halogen lamp 11 is separated into red, green and blue lights by passing through a color liquid crystal shutter 12. The end face of an acrylic rod 13 is irradiated with the lights sequentially with time shifts. The acrylic rod 13, except its light emitting face, is covered with a reflecting foil deposited with aluminum or the like, and has the function of efficiently converting, into linear light, the light entering through the end face of the rod. Consequently, a monochrome liquid crystal shutter array 14 is irradiated with the red, green and blue linear lights sequentially with time shifts.
At this moment, three arrays of pixels corresponding to red, green and blue colors are arranged inside the monochrome liquid crystal shutter array 14, and they each are driven to transmit only the light of a designated color. For example, in the case of the irradiation with the red linear light, only one array of pixels corresponding to the red color can transmit the red linear light while the other two arrays of pixels are kept in a dark state. Each of the red, green and blue linear lights modulated by the monochrome liquid crystal shutter array 14 is focused on a photosensitive sheet 16 by means of a focusing lens array 15. At this time, the photosensitive sheet 16 moves relatively to the monochrome liquid crystal shutter array 14, so as to be exposed at the same position thereof to the red, green and blue linear lights in sequence, thereby obtaining a print image of high quality without any out-of-register colors.
Here, in order to achieve a short printing time, a supertwisted nematic liquid crystal which can respond at high speed of the millisecond order with application of an AC voltage of about 10 kHz is used for the above-described two kinds of liquid crystal shutters (i.e., the color liquid crystal shutter 12 and the monochrome liquid crystal shutter array 14).
According to the conventional optical print head having a configuration as described above, the small-sized color liquid crystal shutter having the red, green and blue filters attached thereto is interposed between the point-source halogen lamp and the acrylic rod for converting the light from the point source into the linear light. The focusing lens array is used to form, on the photosensitive sheet, the linear image formed on the monochrome liquid crystal shutter array, thereby reducing the size and the cost of the optical system for irradiating the photosensitive sheet with the color light emitted from the point-source halogen lamp.
That is, it is sufficient that the color liquid crystal shutter for separating the colors of the white light from the point-source halogen lamp has, at the utmost, such a size as to cover the end face of the acrylic rod. This size is remarkably small in comparison with a printing width of the photosensitive sheet. Furthermore, the constituent elements of the optical print head such as the point-source halogen lamp, and the color liquid crystal shutter for separating the colors, the acrylic rod, the monochrome liquid crystal shutter array and the focusing lens array are arranged in almost close contact with each other along the optical path, so that no special optical path length is required for forming an image or focusing the light. Thus, a very compact color optical print head is realized at a low cost.
However, the conventional optical print head has a problem that there is a large difference in quantity of light exposure of each pixel in the main scanning direction. The quantity of light exposure is more due to the high light intensity nearer the point-source halogen lamp. However, the quantity of light exposure is lower as the light intensity decreases with an increase in the distance from the point-source halogen lamp. As a result, there has arisen the problem of the large difference in quantity of light exposure. Moreover, the use of the acrylic rod and the color liquid crystal shutter has induced a problem of expensiveness of the optical print head.